The present invention relates to DC motor control circuits and, more particularly, DC motor control circuits incorporating current control devices.
A DC motor is typically operated by electrically connecting the motor in series with a DC voltage source to apply a DC voltage across the motor terminals. Additionally, a switch is often electrically connected in series with the motor and voltage source to selectively actuate and deactuate the motor. Such a switch merely permits the motor to be turned off and on and does not allow the motor speed to be varied.
Consequently, DC motor control circuits have been developed to provide a means of controlling the speed of the motor. Probably the simplest of these control devices is a rheostat connected in series with the voltage source and the motor. Because the motor generally operates at a speed proportional to the voltage applied across the motor, the speed at which the motor operates may be varied by varying the resistance provided by the rheostat. If the rheostat is set to provide its minimum resistance, relatively little voltage is lost across the rheostat and a maximum voltage is applied across the motor causing the motor to operate at maximum speed. On the other hand, if the rheostat is set to provide its maximum resistance, a relatively large voltage develops at the rheostat and a relatively small voltage is applied across the terminals of the motor causing the motor to operate at minimum speed.
Solid state control circuits have also been developed to regulate the speed of these motors. Typically, these control devices regulate the speed of the motor by regulating the current, rather than the voltage, supplied to the motor. Because the speed of the motor is also proportional to the current supplied to the motor, current control devices may be utilized to regulate motor speed. Such devices typically have three terminals--a current input terminal, a current output terminal and a control input terminal. Usually, a current control device is connected by the current input and current output terminals in series with the motor to provide current to the motor. Further, the control input terminal of the semiconductor device is typically connected in parallel with the motor to the voltage source. Consequently, the biasing current flowing into the control input terminal bypasses the motor resulting in wasted current and energy since no work is realized from the biasing current. Further, in such a configuration a resistor having a relatively large resistance must be inserted in series between the voltage source and the control input terminal to limit the current entering that terminal to a reasonable value. This results in further complexity and cost of the circuitry as well as additional power dissipation.